Figure 2-9 is a simplified flow diagram of the error correction process. The process consists of three cycles.
1. Read the record in the forward direction and get a eRe error.
2. Read (or space) in reverse over the faulty'record and attempt to compute the track in error.
3. If the track in error was computed (all errors are contained in a single track), make a for-ward read over the faulty record during which error correction is accomplished.
Figure 2-10 is a now diagram of the NRZI error correction algorithm. During a NRZI read forward operation, data characters from tape are input into a eRe generator where a eRe character is devel-oped. At the end of the record, the eRe character read from tape is compared to the generated eRe character. If a match is not obtained, a eRe error is asserted. The program then initiates the error correction process by changing the mode to reverse read or reverse space. The reverse space mode must be selected if it is desired to reverse the tape more than one record, e.g., to move the faulty reco(d over the tape cleaner. If a reverse read was in progress when the eRe error was detected, the mode must be changed to forward read and the faulty record read again. The error correction process can be initiated only when a eRe error is sensed in a forward direction.
As the faulty record is reverse read (or reverse spaced), the track in error is computed. If more than one track is in error, the hardware will not select a faulty track and the error is determined to be uncorrec-table. If the errors are all in the same track, a track in error is computed. In either case, the correction cycle portion of the error correction process is performed.
When the correction cycle is started, the tape must be positioned at the correct location (i.e., at the faulty record) so that the error correction is performed on the proper record. If the read reverse mode was chosen in the preceding paragraph, only one record (the faulty record) can be reverse read. An attempt to read reverse more than one record causes the hardware to abort the error correction pro-cess. If the space reverse mode was chosen, any number of records may be spaced while the error correction process is put on "hold." If the program spaced N records in reverse, it must space N-l records forward before read forward is asserted. (The assertion of read forward initiates the correction cycle portion of the process.) Otherwise, the hardware will attempt the error correction on the wrong record.
I f the error is uncorrectable, no correction will be accomplished during the correction cycle. The faulty record is reread and eRe error is again asserted. In this case, software retry procedures or other corrective action the program may direct will be instituted.
READ FORWARD OVER FAULTY RECORD AND GET CRC ERROR
READ/SPACE REVER$E OVER FAULTY RECORD AND COMPUTE
"TRACK IN ERROR"
READ FORWARD OVER FAULTY RECORD AND PERFORM CORRECTION
DONE
NO
UNCORRECTABLE ERROR
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Figure 2-9 Simplified NRZI Error Correction Flow Diagram
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GO BACK OVER FAULTY RECORD AND COMPUTE WHICH TRACK IS IN ERROR
YES
CHANGE TO FORWARD
Figure 2-10 NRZI Error Correction Flow Diagram
If an erroneous track was detected, correction is accomplished during the correction cycle. This is done by checking each character for vertical parity. Any character having a vertical parity error has the bit corresponding to the track in error complemented. Thus, any number of errors are corrected so long as they all occur in the same track.
When the end of record is sensed, the error correction process is terminated and the reading operation is continued. It is significant to note that the
eRe
character is treated as part of the record and, therefore, is checked for vertical parity an~, if necessary, corrected.2.S PROGRAMMING NOTES
Points to consider in programming the TM03 formatter are listed below.
2.5.1 NRZI Error Correction
I.
If in reverse read when aeRe
error is sensed, the mode must be changed to forward read.The error correction process is entered only when a
eRe
error is sensed in a forward direc-tion.2.
When a forwardeRe
error is sensed, the direction must be changed to reverse to initiate the error correction process.3. If reverse read is used in item 2 above, only one record can be read and then the direction must be changed to forward re.ad.
4. If reverse space is used in item 2 above, any number of records (N) may be reverse spaced;
however, N-I records must be forward spaced before forward read is asserted.
5. If the error is uncorrectable, the program must dictate any corrective action.
2.5.2 Auto Density Select
1. The program does not control the read mode (d~nsity). The presence or absence of an ID burst on the tape selects either PE read or NRZI read for the entire tape.
2. The program can select either write PE or write NRZI at BOT but cannot switch modes at a later point on the tape.
3. If the mode (density) is changed and a rewind off-line command is given, a nonexecutable function error will result.
2.5.3 Other Notes
1. All slaves interfacing to a particular TM03 must be operating at the same speed.
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